In the chemical industry and other industries, overpressure relief systems are the requirement of almost every pressurized line, tank, or other vessel. The most common types of relief devices are rupture discs and relief valves. Rupture discs present an inherent problem in that once ruptured, the disc must be replaced in order for the relief device to seal the vent line associated therewith. If the material in the line or tank is toxic or otherwise hazardous, the line or tank must often be completely emptied and purged before the ruptured disc may be replaced. In addition, if the material in the line or tank is volatile in nature, a substantial amount of the material may escape into the vent system before the ruptured disc can be replaced.
Relief valves overcome the problem associated with ruptured discs somewhat in that valves are biased to close and thereby occlude the vent line after the pressure in the system returns to an acceptable level. However, relief valves are relatively expensive compared to rupture discs. In addition, relief valves require a great deal of maintenance in order to insure their workability and, therefore, must frequently be removed from the process and tested. Such removal again requires that volatile or hazardous materials be removed from the process tanks and lines protected by the relief valves before the valve can be removed. It is, therefore, desirable that multiple relief devices be provided for a common vent such that the vent may normally be in flow communication with the first relief device and, should the first device need maintenance and/or replacement, be able to switch to a second relief device while isolating the first device from the system. In switching from one relief device to another, it is extremely important to insure that the process under pressure is at all times protected by at least one of the relief devices. After the first disc has ruptured, it is desirable for the valve to communicate the pressurized fluid in the line or tank to the unruptured relief device as soon as the pressure within the process returns to a non-overpressure state and stabilizes. In this manner, the loss of material from the process through the ruptured relief device beyond that which is necessary to return the process to a non-overpressure state is minimized.